It is only possible to assign Members with substantive roles as line managers within Compass and not those in acting roles. For example, it will let you assign a GSL but not an acting GSL or a DC but not an acting DC. Unfortunately this is a limitation of the system.
If they have an acting role, please keep a local record of who the line manager is. You do not need to record a line manager on Compass in order to make a role full.